1. Field
The present disclosure generally relates to acoustic treatments for reducing noise, and deals more particularly with septumization of honeycomb sandwiches used as acoustic liners.
2. Background
Aircraft engines produce noise primarily due to rotating blades which compress the airflow as well as rotating blades which extract energy from the airflow and the high-speed airflow through the exhaust nozzles. In order to reduce noise and satisfy noise regulations governing commercial aircraft, aircraft engines may incorporate acoustic panels in various parts of the engine, such as in the nacelle inlets, as well as the aft bypass duct and primary nozzle. These acoustic panels, sometimes referred to as acoustic treatments or acoustic liners, may comprise a honeycomb core sandwiched between a perforated inner skin and a non-perforated outer skin. The honeycomb core often has a middle layer of porous material called a septum which is used to increase the acoustic performance of the liner. The design parameters of the septum in the cells of the honeycomb core usually consist of the porosity of the layer as well as the depth or location relative to the perforated inner skin. The cavities that are formed by the septum act as Helmholtz resonators which attenuate the engine noise.
There are at least three known methods for septumization of honeycomb cores. The first method requires splitting the core and using an adhesive to bond a septum layer between the split cores. Splitting the core to allow a septum layer to be installed generally decreases the mechanical performance of the core, making this septumization technique unsuitable for some applications. The second method involves a lost wax process which creates a buried septum, where a honeycomb core is pressed into wax. A thin layer of liquid resin floats on top of the wax which is then cured to form a solid layer and the wax is melted out of the honeycomb core. This buried septum process requires an extra step in which a laser is used to perforate the solid septum layer to the desired porosity. The third method involves using an automated robotic process where individual septum pieces with tabs are inserted into each honeycomb cell. The tabs of the individual septum pieces are bonded to the honeycomb cell walls by carefully dipping the honeycomb panel to a certain depth into a liquid adhesive. The process of installing the individual septum pieces can be time consuming and expensive. Furthermore, this process is generally restricted to honeycomb with hex-shaped cells which can sometimes be restrictive to the nacelle design due to the potential difficulty in forming a shape with a small radius of curvature.
Accordingly, there is a need for a method of septumizing a honeycomb core used in an acoustic sandwich structure which obviates the need for installing individual septum pieces in the core cells, or splitting the core to allow the installation of a septum layer, or the many steps in using a lost wax process to create a solid septum layer and then using a laser to perforate the solid septum layer.